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.\"     @(#)spp.4	8.2 (Berkeley) 4/19/94
.\"
.Dd April 19, 1994
.Dt SPP 4
.Os
.Sh NAME
.Nm spp
.Nd Xerox Sequenced Packet Protocol
.Sh SYNOPSIS
.In sys/socket.h
.In netns/ns.h
.In netns/sp.h
.Ft int
.Fn socket AF_NS SOCK_STREAM 0
.Ft int
.Fn socket AF_NS SOCK_SEQPACKET 0
.Sh DESCRIPTION
The
.Tn SPP
protocol provides reliable, flow-controlled, two-way
transmission of data.  It is a byte-stream protocol used to
support the
.Dv SOCK_STREAM
abstraction.
.Tn SPP
uses the standard
.Tn NS Ns (tm)
address formats.
.Pp
Sockets using the
.Tn SPP
protocol are either
.Dq active
or
.Dq passive .
Active sockets initiate connections to passive
sockets.  By default
.Tn SPP
sockets are created active; to create a
passive socket the
.Xr listen 2
system call must be used
after binding the socket with the
.Xr bind 2
system call.  Only
passive sockets may use the
.Xr accept 2
call to accept incoming connections.  Only active sockets may
use the
.Xr connect 2
call to initiate connections.
.Pp
Passive sockets may
.Dq underspecify
their location to match
incoming connection requests from multiple networks.  This
technique, termed
.Dq wildcard addressing ,
allows a single
server to provide service to clients on multiple networks.
To create a socket which listens on all networks, the
.Tn NS
address of all zeroes must be bound.
The
.Tn SPP
port may still be specified
at this time; if the port is not specified the system will assign one.
Once a connection has been established the socket's address is
fixed by the peer entity's location.   The address assigned the
socket is the address associated with the network interface
through which packets are being transmitted and received.  Normally
this address corresponds to the peer entity's network.
.Pp
If the
.Dv SOCK_SEQPACKET
socket type is specified,
each packet received has the actual 12 byte sequenced packet header
left for the user to inspect:
.Bd -literal -offset indent
struct sphdr {
	u_char		sp_cc;	/* connection control */
#define	SP_EM	0x10		/* end of message */
	u_char		sp_dt;	/* datastream type */
	u_short		sp_sid;
	u_short		sp_did;
	u_short		sp_seq;
	u_short		sp_ack;
	u_short		sp_alo;
};
.Ed
.Pp
This facilitates the implementation of higher level Xerox protocols
which make use of the data stream type field and the end of message bit.
Conversely, the user is required to supply a 12 byte header,
the only part of which inspected is the data stream type and end of message
fields.
.Pp
For either socket type,
packets received with the Attention bit sent are interpreted as
out of band data.  Data sent with
.Dq send(..., ..., ..., Dv MSG_OOB )
cause the attention bit to be set.
.Sh DIAGNOSTICS
A socket operation may fail with one of the following errors returned:
.Bl -tag -width [EADDRNOTAVAIL]
.It Bq Er EISCONN
when trying to establish a connection on a socket which
already has one;
.It Bq Er ENOBUFS
when the system runs out of memory for
an internal data structure;
.It Bq Er ETIMEDOUT
when a connection was dropped
due to excessive retransmissions;
.It Bq Er ECONNRESET
when the remote peer
forces the connection to be closed;
.It Bq Er ECONNREFUSED
when the remote
peer actively refuses connection establishment (usually because
no process is listening to the port);
.It Bq Er EADDRINUSE
when an attempt
is made to create a socket with a port which has already been
allocated;
.It Bq Er EADDRNOTAVAIL
when an attempt is made to create a
socket with a network address for which no network interface
exists.
.El
.Sh SOCKET OPTIONS
.Bl -tag -width SO_DEFAULT_HEADERS
.It Dv SO_DEFAULT_HEADERS
when set, this determines the data stream type and whether
the end of message bit is to be set on every ensuing packet.
.It Dv SO_MTU
This specifies the maximum amount of user data in a single packet.
The default is 576 bytes - sizeof(struct spidp).  This quantity
affects windowing \- increasing it without increasing the amount
of buffering in the socket will lower the number of unread packets
accepted.  Anything larger than the default will not be forwarded
by a bona fide
.Tn XEROX
product internetwork router.
The data argument for the setsockopt call must be
an unsigned short.
.El
.Sh SEE ALSO
.Xr intro 4 ,
.Xr ns 4
.Sh HISTORY
The
.Nm
protocol appeared in
.Bx 4.3 .
.Sh BUGS
There should be some way to reflect record boundaries in
a stream.
For stream mode, there should be an option to get the data stream type of
the record the user process is about to receive.